Laurentian origin of the Ordovician Grangegeeth volcanic arc terrane, Ireland
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Abstract: U–Pb ages determined by laser-ablation multi-collector inductively coupled plasma mass spectrometry on 199 zircons from a lithic pumice tuff are the first isotopic ages from the Grangegeeth volcanic terrane of Ireland. The youngest zircon population ( n = 24) yields an age of 465.4 +2.4/−3.1 Ma. Inherited zircons yield ages of c . 1120–1520, c . 1650–1914 and c . 2540–2890 Ma. The absence of zircons between c . 550 and 700 Ma argues against involvement of Avalonia as a source of inherited crystals. Overall, the inherited age spectra are characteristic of Laurentian crust or metasedimentary successions of peri-Laurentian provenance. These findings suggest that the Grangegeeth volcanic terrane does not represent a continuation of the intra-Iapetus Exploits arc of Newfoundland. Supplementary material: Full analytical protocols, applied data corrections, processing and plotting rationales and data table for zircon LA-MC-ICP-MS analysis are available at http://www.geolsoc.org.uk/SUP18399 .Keywords:
Volcanic arc
New U-Pb (zircon) data from the northern Boisdale Hills show that rhyolite on Long Island and syenogranite of the nearby Mount Cameron pluton have similar ages of SOS ± 3 Ma and 509 ± 2 Ma, respectively. These ages are similar to those known or inferred from some felsic piutonic units elsewhere in central Cape Breton Island and suggest that latest Cambrian to earliest Ordovician igneous activity was widespread in the Bras d'Or terrane. The new dates also suggest that stratigraphic and structural relationships in the Boisdale Hills arc more complex than previously interpreted.
The dated rhyolite and other volcanic and sedimentary rocks in the northern Bourinot belt in the Boisdale Hills were previously included in undivided Middle Cambrian Bourinot Group. In the southern Bourinot belt, the Bourinot Group was subdivided into the Eskasoni, Dugald, and Gregwa formations. The Eskasoni Formation is dominantly a bimodal volcanic suite with petrological characteristics indicative of origin in a continental within-plate tectonic setting. Its present contacts, both with adjacent older metamorphic and plutonic rocks and with the apparently overlying fossiliferous Dugald and volcanogenic Gregwa formations, are faulted. Our new mapping demonstrates that the Eskasoni, Dugald and Gregwa formations can be extended into the central Bourinot belt where they were previously undivided. However, continuity cannot be demonstrated between these Middle Cambrian units and the dated volcanic and associated sedimentary units in the northern Bourinot belt Hence, assuming that both U-Pb and fossil ages are correct, our interpretation is that the Upper Cambrian -Lower Ordovician Northern Boisdale Hills volcanic unit is younger than the Bourinot Group, although petrochemical data suggest that it formed in a similar tectonic regime.
The presence in the Bourinot belt of fauna characteristic of the Acado-Baltic faunal province appears to tie the Bras d'Or terrane to other Avalonian (peri-Gondwanan) terrenes. However, the Bras d'Or terrane differs from the adjacent Mira terrane which includes Lower as well as Middle and Upper Cambrian units and lacks volcanic and plutonic rocks of this age.
RÉSUMÉ
De nouvelles données U-Pb (zircon) provenant de la partie nord des collines Boisdale montrent que la rhyolite sur l'ile Long et le syénogranite du pluton du mont Cameron à proximité ont des âges similaires, de 505 ± 3 Ma et 509 ± 2 Ma respectivement. Ces âges sont similaires à ceux connus ou supposés pour quelques unités plutoniques felsiques ailleurs dans le centre de l'ile-du-Cap-Breton, et suggèrent que l'activité ignée de la fin du Cambrien au début de l'Ordovicien etait répandue dans le terrain de Bras d'Or. Ces nouvelles datations suggèrent aussi que les relations stratigraphiques et structurales dans les collines Boisdale sont plus complexes que ne le montraient les interprétations antérieures.
La rhyolite datée et d'autres roches volcaniques et sédimentaires de la partie nord de la ceinture de Bourinot dans les monts Boisdale étaicnt auparavant incluses dans le Groupe de Bourinot non divisé, du Cambrien moyen. Dans la partie sud de la ceinture de Bourinot, le Groupe de Bourinot était divisé en formations d'Eskasoni, de Dugald et de Gregwa. La Formation d'Eskasoni est principalement une suité volcanique bimodale avec des caractéristiques pétrologiques indiquant un environnement tectonique intracontinental. Ses contacts actuels, à la fois avec les roches métamorphiques et plutoniques plus vieilles et avec les formations apparemment susjacentes de Dugald, fossilifere, et de Gregwa, volcanique, sont failles. Notre nouvelle cartographic montre que les formations d'Eskasoni, de Dugald et de Gregwa peuvent être distinguees dans le centre de la ceinture de Bourinot où elles ne l'étaient pas auparavant. Toutefois, on ne peut pas démontrer de continuité entre ces unités du Cambrien moyen et les volcanites datées et les unités sédimentaires associées dans la partie nord de la ceinture de Bourinot. Ainsi, en assumant que les ages U-Pb et palcbntologiques sont corrects, notre interprétation est que cette unité volcanique du Cambrien tardif - Ordovicien inférieur du nord des collines Boisdale soit plus jeune que le Groupe de Bourinot, quoique les données pétrologiques suggèrent qu'elle se soit formée dans un environnement tectonique similaire.
La présence dans la ceinture de Bourinot de fossiles caractéristiques de la province faunique acado-baltique semble relier le terrain de Bras d'Or aux autres terrains avaloniens (péri-Gondwana). Toutefois, le terrain de Bras d'Or diffère du terrain adjacent de Mint qui comprend des unités du Cambrien infeérieur autant que moyen et supérieur et ne présente pas de roches volcaniques et plutoniques de cet âge.
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The Taku terrane consists of metamorphosed Carboniferous through Triassic marine clastic strata, volcanic rocks, and limestone that occur along the western margin of the Coast Mountains in southeastern Alaska. These rocks are juxtaposed along mid-Cretaceous thrust faults over Jura-Cretaceous basinal strata of the Gravina belt to the west and beneath Proterozoic through Carboniferous metamorphic rocks of the Yukon–Tanana terrane to the east. This paper presents U–Pb ages and Hf isotope analyses of detrital zircons from the Taku terrane, and compares these values with information from the adjacent Wrangellia, Alexander, and northern and southern portions of the Yukon–Tanana terrane (YTTn and YTTs). These comparisons suggest that (i) Carboniferous strata of the Taku terrane were shed mainly from mid-Paleozoic igneous rocks of YTTs, (ii) Permian strata of the Taku terrane were shed from mid-Paleozoic igneous rocks and intraformational Lower Permian volcanic rocks of YTTs as well as Upper Permian volcanic rocks exposed in YTTn, and (iii) Triassic sandstones were shed from mid-Paleozoic igneous rocks of YTTs, whereas conglomerates were shed mainly from mid-Paleozoic arc rocks in YTTn. Hf isotope analyses of Paleozoic zircons record increasing continental input during Silurian–Devonian and Permian phases of magmatism. Similarities in isotopic characteristics, combined with stratigraphic and geochemical information presented by previous workers, suggest that strata of the Taku terrane accumulated on (and partly as lateral equivalents of) rocks of YTTs, and that the combined assemblages formed outboard or along strike of YTTn.
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Abstract A distinctive Late Ordovician volcano-sedimentary terrane, embracing the Carriers Well Formation and Everetts Creek Volcanics and dismembered slivers now structurally intercalated in the adjoining Wairuna Formation, is located within the Broken River Province of the northern Tasmanides. It abuts a basement of mafic–ultramafic rocks (Gray Creek Complex) and overlying Early Ordovician deep marine sedimentary and volcanic strata (Judea Formation) which host tonalitic plutons. The terrane lies at the western, inboard-margin of the Camel Creek Subprovince, a broad tract of multiply deformed mid-Paleozoic turbidites with minor basalt and chert variously interpreted as the infill of a backarc basin or an accretionary wedge. U–Pb dates from detrital zircon indicate a maximum Late Silurian age for siliciclastic rocks from the previously undated Wairuna Formation. Geochemistry of volcanic rocks from the volcano-sedimentary terrane show them to be largely of mafic to intermediate compositions of calc-alkaline affinity, comparable with broadly coeval Macquarie Arc volcanic suites of the southern Tasmanides. Trace-element systematics identify a subduction relationship for the volcanic suite and V/Ti employed as a discrimination tool identifies the terrane as representing an oceanic island arc, consistent with its sedimentary facies which include volcaniclastic mass flow deposits, limestone, and radiolarian chert. Continent-derived sandstone in the sedimentary assemblage, confirmed by the ages of detrital zircon from a sandstone sample from the Carriers Well Formation, indicates that the oceanic island arc developed proximal to the Late Ordovician continental margin of East Gondwana. Its nature and location bear on the tectonic setting of the entire Camel Creek Subprovince, for which interpretation as an Early Silurian–Early Devonian accretionary wedge is favoured. Collision of the island arc with the continental margin, and associated deformation of part the intervening oceanic crustal tract, now represented by the Gray Creek Complex and its sedimentary cover (Judea Formation) registers the initiation of subduction accretion in late Early Silurian (Llandoverian) time. It marks early-stage orogenesis in the Broken River Province, accurately timed by stratigraphic relationships in the basinal succession developed in the Graveyard Creek Subprovince immediately to the west of the arc assemblage. Tectonism was regionally developed in north Queensland at this time, coeval with the Benambran Orogeny of the Lachlan Orogen in which the Macquarie Arc was likewise accreted to the East Gondwana margin. Benambran orogenesis marks a general phase of shortening, and removal by subduction, of oceanic crust and inversion of continent-derived overlying sedimentary cover along the East Gondwana margin. Key Words: Benambran OrogenyBroken River ProvinceEast Gondwanaoceanic island arccollisional accretionOrdovicianTasman Orogenic Zone Acknowledgements Assistance from Sebastien Meffre in U–Pb analyses of zircons and geochronological determinations, and from Adella Edwards in drafting the figures, is gratefully acknowledged, as are constructive manuscript reviews from Graziella Caprarelli and Russell Korsch. The Heath family of Lucky Springs is thanked for access to Lucky Springs and support for fieldwork. The work was partially funded by ARC Linkage Grant LP0884011.
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Geological relationships preserved in the New Brunswick segment of the Appalachian orogen are key to deciphering the complex tectonic events that occurred during the closing of the Paleozoic Iapetus Ocean. These events can be explained in terms of geodynamic interactions between eight lithotectonic terranes. The first, the Caledonia terrane, comprises Neoproterozoic volcanic arc sequences and comagmatic plutons considered to form part of the microcontinent of Avalonia. The seven other terranes are associated with the microcontinent of Ganderia and consist of (1) Brookville terrane–Mesoproterozoic to Neoproterozoic platformal carbonates and Neoproterozoic to Early Cambrian plutonic rocks, (2) New River terrane–Neoproterozoic volcanic arc sequences and comagmatic plutons overlain by a Cambrian Penobscot volcanic arc sequence, (3) Annidale terrane–Upper Cambrian to lower Lower Ordovician Penobscot volcanic arc-backarc sequence and unconformably overlying upper Lower Ordovician volcanic sequence, (4) St. Croix terrane–Cambrian to Upper Ordovician sedimentary sequence deposited on the passive margin of the New River terrane, (5) Miramichi terrane–Cambrian to Lower Ordovician sedimentary sequence and unconform-ably overlying Middle to Upper Ordovician Tetagouche volcanic backarc sequence; and Lower to Middle Ordovician Meductic volcanic arc sequence, (6) Elmtree terrane–Middle to Upper Ordovician backarc ophiolitic sequence, and (7) Popelogan terrane–Middle to Upper Ordovician volcanic arc sequence.
In New Brunswick, closure of the Iapetus Ocean is attributed to four major tectonic episodes: (1) the Penobscot orogeny, which accreted the Miramichi, Annidale, and St. Croix terranes to the New River terrane on the trailing edge of Ganderia by closing a Penobscot backarc basin in the Early Ordovician, (2) the Taconic orogeny, which accreted the Popelogan terrane on the leading edge of Ganderia to the Laurentian margin in the Late Ordovician and effectively closed the main tract of the Iapetus Ocean, (3) the Salinic orogeny, which accreted the Elmtree and Miramichi terranes to the Laurentian margin by closing the Tetagouche backarc basin in the Silurian, and (4) the Acadian orogeny, which accreted the Caledonia terrane (Avalonia) to the Brookville and New River terranes on the trailing edge of Ganderia in the latest Silurian to earliest Devonian and in the process closed the last remaining oceanic tract in the northeastern Appalachians.
RÉSUMÉ
Les corrélations géologiques préservées dans la partie du Nouveau-Brunswick de l’orogène des Appalaches sont essentielles pour bien comprendre le déroulement des événements tectoniques complexes survenus au moment de la fermeture de l’océan Iapetus, pendant le Paléozoïque. Il est possible d’expliquer ces événements par des interactions géodynamiques entre huit terranes lithotectoniques. Le premier, soit le terrane calédonien, comprend des séquences d’arc volcanique et des plutons comagmatiques du Néoprotérozoïque dont on estime qu’ils faisaient partie du micro-continent Avalonia. Les sept autres terranes sont associés au microcontinent Ganderia et comprenaient les terranes que voici : (1) le terrane Brookville – formé de carbonates de plate-forme du Mésoprotérozoïque au Néoprotérozoïque et de roches plutoniques du Néoprotérozoïque au début du Cambrien; (2) le terrane New River – formé de séquences d’arc volcanique du Néoprotérozoïque et de plutons comagmatiques recouverts par une séquence d’arc volcanique de Penobscot du Cambrien; (3) le terrane Annidale – formé d’une séquence d’arc-arrière-arc volcanique de Penob-scot du Cambrien supérieur au début de l’Ordovicien inférieur, qui repose en discordance sur une séquence volcanique de la fin de l’Ordovicien inférieur; (4) le terrane Ste‑Croix – formé d’une séquence sédimentaire du Cambrien à l’Ordovicien supérieur, déposée sur la marge passive du terrane New River; (5) le terrane Miramichi – formé d’une séquence sédimentaire du Cambrien à l’Ordovicien inférieur, qui repose en discordance sur une séquence d’arrière-arc volcanique de Tetagouche de l’Ordovicien intermédiaire à supérieur, et sur une séquence d’arc volcanique de Meductic de l’Ordovicien inférieur à intermédiaire; (6) le terrane Elmtree – formé par un cortège d’arrière-arc ophiolitique de l’Ordovicien intermédiaire à supérieur; et (7) le terrane Popelogan – formé par une séquence d’arc volcanique de l’Ordovicien intermédiaire à supérieur.
Au Nouveau-Brunswick, la fermeture de l’océan Iapetus est attribuée à quatre épisodes tectoniques majeurs : (1) l’orogenèse Penobscot, qui a formé le terrane New River par accrétion des terranes Miramichi, Annidale et Ste‑Croix, sur le flanc résiduel de Ganderia, par la fermeture du bassin marginal de Penobscot au début de l’Ordovicien; (2) l’orogenèse taconique qui a formé le terrane Popelogan par accrétion sur la marge laurentienne de la partie frontale de la nappe de Ganderia à la fin de l’Ordovicien, ce qui a effectivement ceinturé définitivement la principale étendue de l’océan Iapetus; (3) l’orogenèse salinique, qui a formé par accrétion les terranes Elmtree et Miramichi sur la marge laurentienne, par la fermeture du bassin marginal de Tetagouche au cours du Silurien; et (4) l’orogenèse acadienne, qui a formé le terrane calédonien (microcontinent Avalonia) par accrétion sur les terranes Brookville et New River, sur le flanc résiduel de Ganderia, entre la fin du Silurien et le début de Dévonien; au cours de cette période, les dernières traces d’océan ont disparu du nord-est des Appalaches.
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There are several clay beds recognized from the Wufeng Formation,the top of the Ordovician and,Lungmachi Formation,the bottom of the Silurian in the Yangtze Block.These clay beds were taken as K-bentonites,which was produced by volcanic ash-fall deposits after diagenic metamorphism in marine environments.These K-bentonites are widely distributed along the margin of the Yangtze Block,such that can provide important constraints on the plate tectonic setting of the Yangtze Block as well as major geological events in the Early Paleozoic.In this contribution,we carried our mineralogical and geochemical studies on K-bentonite samples collected from two Ordovician-Silurian boundary sections,located in Yichang,Hubei Province and Tongzi, Guizhou Province.The results show that in addition to clay minerals,the clay beds also content quartz,feldspar,biotite,apatite, zircon,etc.,which are all common minerals in intermediate to silicic volcanic rocks.These clay beds should be typical K-bentonite. Most of the K-bentonite samples show typical characteristics of arc volcanic rocks,which were probably related to the subduction and closure of the Qinling Ocean in the Early Paleozoic.Alternatively,there might have been another subduction to the southeast of the South China Block.Sample YC0711 have no obvious Nb anomaly,but show Ti negative anomaly.They fall close to arc fields in a Th/ Yb-Nb/Yb diagram,suggesting that the protoliths were likely to special types of arc rocks,e.g.,Nb-enriched basalt.The peak age of the volcanic activity was Hirnantia stage,which is very close to the mass extinction at Ordovician-Silurian boundary,the second largest mass extinction in the Earth's history,which was generally attributed to the Gondwana glaciation.Considering that K-bentonite is widely distributed in South China,North America and Europe,the volcanic event at the Ordovician-Silurian boundary was a global event.In addition,most of these volcanic rocks are silicic,which are usually volatile enriched,and thus might have major effects on the atmosphere and biosphere.Here we propose that these volcanic events were the triggers of mass extinction at the Orodocician-Silurian boundary as well as the Gondwana glaciation.
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Abstract The Cambrian succession of the Takaka Terrane represents a volcano‐sedimentary arc complex, which was once part of the Pacific margin of Gondwana. The succession has been folded and disrupted by faulting during at least four tectonic events, bringing together at least 12 fault slices with dissimilar stratigraphy. A revised stratigraphy and regional models for lithofacies relationships, magmatic history, and tectonic setting are proposed. They are based on analysis of the lithostratigraphy, biostratigraphy, petrology, geochemistry, geochronology, and field relationships. Volcanic rock formations recognised are Benson Volcanics (arc), Mataki Volcanics (back‐arc), and Cobb Igneous Complex (boninites), which are all grouped in the Devil River Volcanics Group. Petrography and trace element geochemistry enable discrimination of the volcanic units of the Devil River Volcanics. Rocks of the Benson Volcanics are by far the most voluminous and are subdivided into nine (informal) volcanic suites. Arc activity lasted from middle Mid Cambrian (Undillan) to Late Cambrian ( Mindyallan or younger), about 20 m.y., constrained by ages based on trilobites, brachiopods, and conodonts in mterbedded sediments and U‐Pb and Ar‐Ar dates. This period of arc activity is different from those in Tasmania and southeast Australia, where post‐collisional volcanism commenced in the late Mid Cambrian. A separate igneous unit in the Takaka Terrane is the rift‐related Gendarme Dolerite of latest Cambrian to early Ordovician age. Sedimentary formations recognised are Tasman Formation, Mount Benson Sandstone (new), Ruby Saddle Formation (new), Lake Peel Formation (new), Lockett Conglomerate, Christmas Conglomerate, Salisbury Conglomerate, and Heath Creek beds, all comprising the Haupiri Group, and the Junction Formation (group unassigned). Rocks of the Haupiri Group were deposited in a back‐arc or intra‐arc setting.
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Several Ordovician volcanic terranes are preserved between the Paleozoic North American margin and the Avalon Terrane in the northern Appalachians of Maine. They have been interpreted as remnants of an island arc on the basis of their current position in the Appalachians or as ocean islands near the margin of Avalon on the basis of paleontologic data. A study of one of these terranes, the Lunksoos terrane in northern Maine, was undertaken through a combination of paleomagnetic and structural analyses in order to understand its tectonic evolution. Paleomagnetic study of the pre‐Caradoeian (pre‐Late Ordovician) Stacyville volcanics revealed three components of magnetization. The final magnetization removed (D=90°, I=36°, α95=11.8°) is interpreted to be primary and yields a paleolatitude of approximately 20°S; however, a Late Ordovician remagnetization cannot be excluded. The Stacyville volcanics now overlie a Cambrian and Lower Ordovician sequence (the Grand Pitch and Shin Brook formations) that is believed to have been deposited at the southern side of Iapetus, possibly near the margin of Avalon. During the Middle and Late Ordovieian, Avalon was at paleolatitude s higher than 40°S, while the volcanics were either formed or remagnetized at a paleolatitude of 20°S. The pre‐Caradocian Stacyville volcanics therefore could not have been deposited directly on the Grand Pitch‐Shin Brook sequence but must have been tectonically emplaced. The Rockabema Diorite which intrudes the entire Cambro‐Ordovician sequence constrains the timing of this emplacement to be pre‐late Early Silurian. These relationships indicate that the Lunksoos area is a composite terrane within the Maine Appalachians. The tectonic emplacement scenario is unnecessary if the Grand Pitch and Shin Brook formations were not located on the south side of Iapetus during the Ordovician. This alternative, however, poses conflicts with the interpretation of faunal affinities.
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Terrigenous sediment
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